Connection problem with gsm etc.

Connection problem with gsm etc.

I received my new M8 on 16/05 and everything was great until 21/05 when I could not make a call, receive a call send sms or receive sms or run ussd message for balance check even though the phone was showing a full service provider signal. I restarted the phone and on the 10th restart could make one or two calls, send a couple of sms then back to the problem. Again after restarting numerous times i could make a couple of calls etc. Then after restarting the phone nothing.

I did a factory re-set and for a while (1, 2 hours) ok then the problem re occurred. I swapped out the sim with 2 others still the same. I did another factory re-set still the same problem and now once in 10, 15 restarts I can make a couple of calls etc.

The phone is unlocked from HTC.

I have searched for a similar situation here for some helps, tips etc but can not find anything.